The study of the interaction of polycyclic aromatic carcinogens and allied compounds with nucleic acids, nucleotides, polynucleotides and proteins under controlled laboratory conditions is proposed. The spectroscopic flash photolysis technique will be employed in which the decay of the transient triplet state of the aromatic hydrocarbon will be monitored. The triplet state is a sensitive probe of its microenvironment, particularly to local physical interactions, the viscosity and the presence of quenching molecules such as oxygen and certain inorganic ions. Lifetime studies in the presence and absence of quenching species thus provide information about the accessability of the polycyclic aromatics when complexed with nucleic acids or proteins. These triplet-triplet absorption studies will also be extended to unicellular in vivo systems such as E. Coli and Paramecium. By comparing the in vivo and in vitro studies information about the mode of action and reactivity of the sensitizers in the photodynamic effect will be sought. For in vivo experiments the flash photolysis technique will be complemented by fluorescence studies, using the sensitive single photon counting technique. The latter will be used to determine the particular cellular components which preferentially solubilize the sensitizers in the photodynamic effect, and the rate of diffusion of the sensitizer and appropriate fluorescence quenching species. The object of the proposed research is to determine the relationship between molecular structure and photodynamic activity and, if possible, to relate it to the carcinogenic activity of the polycylic compounds.